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When purchasing seeds, annuals, perennials and landscape plants, gardeners often come across tags indicating full sun, partial sun or shade. And sometimes the instructions spread the plant’s light requirements over two designations. Although no hard-and-fast rule exists as to what constitutes full sun, plants that do not receive the recommended minimal number of hours of direct sunlight will not grow well. When choosing locations for plants requiring full sun, gardeners also need to keep soil conditions and microclimates in mind.

Light Requirements

Plants designated as requiring full sun need at least six to eight hours of strong light, while some may need as much as 12. Inexperienced gardeners have the misconception that full sun plants need to be in direct sunlight all day, but this is usually not the case. Full-sun plantings should receive at least some hours of sun in the afternoon when solar radiation is at its strongest. For many plants, obtaining sunlight during morning or early evening hours is also acceptable. Full-sun plants cover a broad palette and include most vegetables, fruits, herbs and bright, showy flowers.

Factors Affecting Intensity

Reflected light from buildings, light-colored garden walls and pavement and even water can increase the intensity of sunlight on plants, making the immediate area hotter. The presence of these factors can make partial sun areas suitable for full-sun plants. Southern and western exposures provide the most sunlight and heat and are often associated with dry soil. Sun-loving plants that are heat- and drought-tolerant perform best in these areas.

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Observe Light Patterns

By making a map of your yard and observing the way light patterns change throughout the day, gardeners can easily determine how much sun a particular area receives. Sun exposure not only changes throughout the day but also seasonally due to the position of the sun relative to the horizon. The presence of trees, buildings, fences and other objects can also affect sun exposure during the day. Understanding how much direct sun exposure an area receives at what time of day is key in selecting proper full-sun plants. The more intense the sunlight, the more photosynthesis that occurs.

Houseplant Considerations

Like their outdoor counterparts, houseplants often are labeled with the same sun requirements. Exposure to light is the most important factor to successfully growing plants indoors. Houseplants labeled for full sun should always receive exposure to a southern window. The light levels in all but the sunniest rooms in a home are usually insufficient for growing full-sun plants. As with outdoor plants, seasonal light also affects intensity. Gardeners can purchase light meters to determine the correct amount of light needed and supplement with artificial lighting.